One-package structural adhesive

Abstract

A one-package primerless structural adhesive is disclosed that is particularly well suited for adhering one or more substrates of fiber-reinforced plastics. This adhesive uses dicyandiamide, its substituted derivatives, or melamine as the principal curative. A polyisocyanate prepolymer is made from various polyols reacted with polyisocyanates. In the absence of moisture, the curative and polyisocyanate can coexist in a single package without reacting for periods of time in excess of four months. Upon heating the adhesive to 150° C., it cures in about five minutes or less, forming an initial structural bond that after heat aging is often stronger than the cohesive strength of the fiber reinforced substrate.

What is claimed is: 
     
       1. An adhesive composition comprising: a) a prepolymer comprising the reaction product of one or more polyols with one or more polyisocyanates wherein the NCO:OH ratio is from about 10:1 to 30:1,   b) a curative comprising melamine and/or cyanoguanidine having from 0 to 2 substituents which are independently alkyls of 1 to 8 carbon atoms or phenyl, said melamine and/or cyanoguanidine of the curative having reactive NH 2  groups, being in particle form, and having at least 98 wt. % of its particles of a particle diameter of less than 25 μm, and   c) one or more mineral fillers   wherein the ratio of the NCO groups of the prepolymer to the NH 2  groups of the curative is from 1.2 to 2.2 and wherein the adhesive composition has the capacity to form a structural bond between two parts in 10 minutes or less at 150° C.   
     
     
       2. An adhesive composition according to claim 1, wherein the prepolymer component is about 80 wt. % or more of the adhesive and said curative comprises cyanoguanidine having neither alkyl nor phenyl substituents and having 98 wt. % of its particles less than 15 μm in diameter. 
     
     
       3. An adhesive composition according to claim 1, wherein said curative comprises cyanoguanidine having neither alkyl nor phenyl substituents. 
     
     
       4. An adhesive composition according to claim 1, wherein said curative comprises melamine. 
     
     
       5. An adhesive composition according to claim 1, wherein said curative comprises a mono or dialkyl substituted cyanoguanidine. 
     
     
       6. An adhesive composition according to claim 3, wherein the one or more polyols have number average molecular weights from about 2,000 to 8,000 and have an average functionality above 2, wherein said mineral fillers include talc, wherein the cure time to form an initial structural bond is about 5 minutes or less at a cure temperature of 150° C., and wherein at least 98 wt. % of said particles are of a diameter of less than 15 μm. 
     
     
       7. An adhesive composition according to claim 6, wherein said one or more polyols are polyols having greater than 80 number percent of their hydroxyl groups being primary hydroxyl groups. 
     
     
       8. An adhesive composition according to claim 7, wherein the polyisocyanates have NCO groups, wherein said NCO groups are pendant to aromatic rings in the polyisocyanate, wherein the ratio of NCO groups in the prepolymer to NH 2  groups in the curative are from about 1.4 to 1.9 and wherein the curative has at least 98 wt. % of its particles of diameter less than 10 μm. 
     
     
       9. An adhesive composition according to claim 8, wherein said one or more polyols are poly(alkylene oxide) based polymers, wherein the alkylene unit has from 1 to 3 carbon atoms, and wherein the curative has at least 98 wt. % of its particles of diameter less than 6 μm. 
     
     
       10. A laminate comprising two or more substrates adhered by said adhesive composition according to claim 1. 
     
     
       11. A laminate according to claim 10, wherein at least one of the substrates is a fiber reinforced thermoset plastic. 
     
     
       12. A laminate according to claim 11, wherein the at least one fiber reinforced thermoset plastic has a primerless surface and said adhesive is applied to said primerless surface. 
     
     
       13. A laminate comprising two or more substrates adhered by said adhesive composition according to claim 6. 
     
     
       14. A laminate according to claim 13, wherein at least one of the substrates is a fiber reinforced thermoset plastic. 
     
     
       15. A laminate according to claim 14, wherein said at least one fiber reinforced thermoset plastic substrate has at least one primerless surface and wherein said adhesive composition is applied to one of said at least one primerless surfaces. 
     
     
       16. A laminate comprising two or more substrates adhered by said adhesive composition according to claim 9, wherein at least one of the substrates is a molded fiber reinforced thermoset polyester part. 
     
     
       17. In a process comprising forming a urethane-based structural adhesive from isocyanate-terminated prepolymers and one or more curatives, wherein said isocyanate terminated prepolymers are the reaction product of one or more polyols and one or more polyisocyanates reacted at an NCO:OH ratio of from 10:1 to 30:1, the improvement wherein said one or more curatives comprise a powdered curative selected from cyanoguanidine, mono or di alkyl and/or phenyl substituted cyanoguanidine, melamine or mixtures thereof; and wherein said powdered curative has a particle size distribution such that 98 wt. % of said powdered curative has a particle diameter of less than 25 μm. 
     
     
       18. In a process according to claim 17, wherein said powdered curative comprises said cyanoguanidine, and where the process includes curing the adhesive at temperatures of 125° C. or higher. 
     
     
       19. In a process according to claim 18, wherein said powdered curative consists essentially of cyanoguanidine with neither alkyl nor phenyl substituents and said powdered curatives are present in an amount of from 1 to 20 parts by wt. per 100 parts by wt. of said prepolymers and wherein the isocyanate terminated prepolymers are made from polyols and aromatic polyisocyanates.